Update to bats-core v1.7.0

[toolmantim]

Bringing #31 back to life, this updates bats-core to v1.7.0

It introduces a necessary breaking change, renaming BATS_PATH to BATS_PLUGIN_PATH, because BATS_PATH is now used internally by bats.

To update tests:

 #!/usr/bin/env bats
 
-load "$BATS_PATH/load.bash"
+load "$BATS_PLUGIN_PATH/load.bash"

@test "prints some output" {
  run $PWD/hooks/command
  assert_output --partial "some output"
  assert_success
}